This invention relates generally to switched capacitor active filters, and more particularly the invention relates to a switched capacitor filter having nonlinear quality coefficient (Q) which permits quality enhancement.
U.S Pat. No. 4,417,214 for "Monolithic IC General Purpose Active Filter" discloses a switched capacitor simulation of an active R-C universal filter. The basic R-C active filter is a hybrid circuit in which three op amps are serially connected as a summer followed by two negative integrators. Four external discrete resistors are user-selected to configure the filter, while three hybrid thin-film resistors and two discrete capacitors, all mounted on a ceramic substrate, establish operating performance of the filter.
The switched capacitor filter simulation eliminates the necessity for using the hybrid resistors and capacitors through use of switched capacitor positive integrators. The filter can be fabricated in a single integrated circuit and experiences a linear quality coefficient, Q. However, the switched capacitors do induce noise in the filter. Further, external resistors are still employed for filter configuration, and the external resistors introduce parasitic capacitance and are limited in tuning accuracy.